With the development of wireless multimedia services, demands for high data rates and user experience are increasing, thus putting forward higher requirements for the system capacity and coverage of traditional cellular networks. The popularity of applications such as social networking, short-distance data sharing and local advertising, on the other hand, increases people's requirements for understanding nearby people or things of interest and communicating with them (Proximity Services). Traditional cell-based cellular network has obvious limitations in supporting high data rates and proximity services, in the context of this demand, the Device-to-Device (D2D) technology, a representative of the new direction of future communication technology development, was introduced. The application of the D2D technology can reduce the burden on the cellular network, reduce battery power consumption of the user equipment, increase the data rate, and improve the robustness of the network infrastructure, which can well meet the requirements of high data rate services and proximity services.
The D2D technology can work on the authorized or unauthorized frequency bands, and allow a plurality of user equipments that support the D2D function (that is, D2D user equipment, D2D UE) performing direct discover/direct communication in the existing network infrastructure or in the case of having no network infrastructure. There are three main D2D application scenarios:
1) the UE1 and the UE2 exchange data under the coverage of cellular networks, and user plane data do not pass through the network infrastructure, as mode 1 shown in FIG. 1;
2) UE relay transmission in a weak/non coverage area, as mode 2 shown in FIG. 1, it allows the UE4 with poor signal quality communicating with the network through the nearby UE3 which has network coverage, so as to help the operator expand coverage and increase capacity;
3) In the event of an earthquake or an emergency and the cellular network cannot work properly, the devices are allowed to communicate directly, as mode 3 in FIG. 1, the control plane and the user plane between the UE5, the UE6 and the UE7 perform one hop or multi-hop data communication without passing through the network infrastructure.
The D2D technology usually comprises the D2D discovery technology and the D2D communication technology:
1) the D2D discovery technology is a technology used to judge/determine that a plurality of D2D user equipments are neighboring with each other (e.g. within the scope of the D2D direct communication) or judge/determine that a first user equipment is neighboring with a second user equipment.
2) the D2D communication technology is a technology in which some or all of the communication data between the D2D user equipments may be directly used for communication without passing through the network infrastructure.
In a scenario of having the cellular network coverage, the D2D discovery or communication resources are usually scheduled and allocated by the base station, which can improve the resource reuse efficiency, and also ensure the control of the network side on the D2D discovery or communication as well as the effect of coordination between the D2D and the cellular system. The discovery or communication resources of the D2D UE can be dynamically scheduled by the base station, or the base station can allocate semi-persistent scheduling resources to the D2D UE. In public security scenarios, the D2D communication requires high robustness, and can still provide services to the maximum extent in the case that the current communication resources are insufficient or congest or the network infrastructure is paralyzing. Therefore, the public security requires the D2D communication works not only in scenarios of having network coverage, but also in scenarios of having some as well as no network coverage. In scenarios of having no network coverage, the D2D user equipment can work in the self-organized manner, or some D2D user equipments can be selected in the scenario of having no network coverage as control nodes to perform the D2D communication management and control, these control nodes provide functionality similar to the base station, such as synchronization and resource management.
A plurality of D2D UEs performing the D2D discovery or communication may be served by different base station (or Control Node), for example, the neighboring UEs performing the D2D communication are respectively located at the coverage edges of different base station, while in the related art, there is no D2D communication resource allocation method for this scenario. Typically, the D2D UEs in the network coverage take the synchronization information sent by the base stations as the D2D synchronization reference, but in a scenario in which the UEs performing the D2D discovery or communication are respectively served by different base stations, different base stations cannot guarantee that they are in the synchronized state (especially for the FDD base station), if the UEs performing the D2D discovery or communication in the scenarios of having no network coverage are respectively served by different control nodes, the different control nodes may not interact in order to be synchronized. In such a scenario, how the UEs performing the D2D discovery or communication are synchronized has no solutions yet.